|  | 5.4 | IGRP Operation
| 5.4.2 | Differentiating interior, system, and exterior routes
Cisco's primary goal in creating IGRP was to provide a robust protocol for routing within an autonomous system (AS). An AS is a collection of networks under common administration sharing a common routing strategy. IGRP uses a combination of user-configurable metrics, including network delay, bandwidth, reliability, and load. IGRP advertises three types of routes: interior, system, and exterior. Interior routes are routes between subnets in the network attached to a router interface. If the network attached to a router is not subnetted, IGRP does not advertise interior routes. Additionally, subnet information is not included in IGRP updates, which poses a problem for discontiguous IP subnets. System routes are routes to other major networks within the AS. The router derives system routes from directly connected network interfaces and system route information provided by other routers that use IGRP. System routes do not include subnetting information. Exterior routes are routes to networks outside the AS that are considered when identifying a gateway of last resort. The router chooses a gateway of last resort from the list of exterior routes that IGRP provides. The router uses the gateway of last resort if it does not have a better route for a packet and the destination is not a connected network. If the AS has more than one connection to an external network, different routers can choose different exterior routers as the gateway of last resort.